Electron emitters are used to generate electrons that are directed into beams for electron microscopy and other applications. Electron microscopy includes scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy, as well as analytical variations of these techniques. An ideal electron source produces a beam of electrons that can be focused to an extremely small spot with sufficient current to provide rapid, consistent data collection. Such an electron source is typically characterized by low energy spread among the emitted electrons, high brightness, and long-term stability.
To be freed from a solid surface, an electron must overcome an energy barrier. The height of this energy barrier is referred to as the “work function” of the material. Thermionic electron emitters are heated by a filament to provide the electrons with sufficient thermal energy to overcome the energy barrier and leave the surface. Schottky electron emitters use a combination of coating materials that lower the work function, heat to provide thermal energy, and an electric field to free the electrons. Cold field electron emitters, on the other hand, use an electric field to provide the conditions for electrons to tunnel through the energy barrier, rather than providing the electrons with the sufficient thermal energy to pass over the barrier.
Because cold field emitters provide high brightness with a small energy spread, they offer improved resolution for electron microscopy. Cold field emitters are not commonly used in electron microscopy, however, because of both long term and short term emission instability. Short term stability refers to the ability to produce a constant emission distribution over a period in which an individual operation, such as forming an image, occurs. Long term stability or source lifetime refers to the ability to provide a relatively constant emission distribution for performing many operations, typically over a period of hours or days.
Although electron beam columns operate in a vacuum, the vacuum is not perfect, and some residual gas molecules are always present. The residual gases tend to adsorb onto the emitter tip, causing changes in the emission characteristics. Moreover, electrons from the emitter collide with the gas molecules, creating positive ions that are accelerated back towards the emitter by the electric field. The impact of these ions damage the emitter surface by sputtering material from the surface, and the damaged surface changes the electron emission characteristics. In Schottky emitters, which typically operate at about 1,800 K, the emitter surface repairs itself over time, as atoms migrate over the surface. This “self-repair” does not occur in cold field electron emitters, which operate at close to room temperature. Cold field electron emitters are therefore heated or “flashed” periodically to allow surface atoms to migrate to repair damage and to remove molecules that are adsorbed onto the emitter surface. Heating the cold field emitter, however, interrupts the operation of electron microscope or other equipment in which the emitter is installed. Cold field emitters can be operated with an external feedback control loop that detects the beam current and maintains a constant beam current by increasing the voltage applied to the emitter as the current decreases over time.
Because cold field emitters rely on a very high electric field to emit electrons from the surface, the emitters typically require a very sharp point, that is, a tip with a very small radius, to achieve the required electric field. The small emitting area of a cold field emitter causes more short term variation in the electron beam because small variations in the tip structure and random motion of adsorbed gases on the tip are not averaged out over a large emitting area. Also, heating the emitter to clean the tip tends to blunt the tip, as atoms in the emitter rearrange themselves to reduce the surface energy. After heating the tip many times, the tip radius increases to a point at which the radius is too large for adequate field emission.
Schottky emitters typically operate at pressures in the 10−9 Torr (1.3×10−9 mbar) range. To improve the stability of cold field emitters, they are typically operated at a pressure of less than 10−10 Torr (1.3×10−10 mbar). The lower pressure reduces the amount of gas that is adsorbed onto the cold field emitter and reduces the damage from ion bombardment, thereby reducing the required frequency of flashing. The lower pressure, however, is more difficult to achieve. Because of the instability of cold field emitters, Schottky emitters, which operate at higher pressures and are more stable, have become the standard electron emitter for most high resolution microscopy systems and applications.